The present invention relates to a cutter shaft for a wood machining apparatus, in which the cutter shaft is provided with a plurality of pockets in each of which a cutting knife is arranged and in which a pressure plate abuts against a side face of each cutting knife which in turn is acted upon by a wedge biased in radial outward direction by a spring and during rotation of the shaft also by centrifugal force.
Such cutter shafts have been built by the Hombak Maschinenfabrik KG, to which the present application is assigned, for about eighteen years. The pressure plate is thereby constructed as knife holder with which the cutting knife is screwed together and adjusted with respect to the desired circle at which the cutting edge of the cutting knife has to be located with respect to the outer periphery of the cutter shaft. The unit comprising the cutting knife and the knife holder is inserted into the pocket of the cutter shaft, after pushing the wedge against the action of the spring in radial inward direction, whereby the knife holder positively engages with an inner edge thereof a holding bar arranged in the pocket and connected by screws or the like to the cutter shaft. Subsequently thereto the wedge is released so that the latter abuts under the pressure of the spring acting thereon, with a wedge face against a corresponding face of the knife holder to press the latter against the cutting knife and the holding bar. The wedge angle in this known construction is about 30.degree.. In this known construction the cutting knife abuts with a side face thereof opposite the side face provided with the cutting edge directly on a face of the pocket provided in the cutter shaft. While during standstill of the shaft the spring abutting against the wedge exerts a pretension, the actual clamping force for holding the cutting knife in a fixed predetermined position during the machining operation is obtained by the centrifugal force acting on the wedge during rotation of the cutter shaft.
This known cutter shaft arrangement has to be reconditioned about once a year due to damage of the outer surface of the cutter shaft at portions thereof which, in the direction of rotation of the shaft, are located rearwardly of the cutting knives. After extended use of the known cutter shaft, the radial outer regions of the cutter shaft body, onto which the cutting knives abut, bulge slightly in outward direction. These damages have been studied for years. According to a generally held opinion this damage was due, on the one hand, to the substantially tangential force component of the wedge and, on the other hand, to the cutting pressure during machining which, especially during engagement of the knives with hard branches, foreign elements, etc. constitutes a high continuously acting stress. The mentioned damage could be reduced, but not completely avoided, by using high-grade steel for the cutter shaft.
The peripheral surface portion of the cutter shaft, located in direction of rotation of the latter, behind the cutting knives are also subjected to considerable wear. This wear leads to groove formation, rearwardly of the above-mentioned upwardly bulging portions, which likewise requires a yearly repair of the shaft.
The present invention is based on the surprising recognition that the above-discussed damage of the cutter shaft is mainly due to the self-locking of the movable wedge. Since rust, dirt and burnt rosin will deposit on the metallic slide faces of pressure plate and wedge, the friction coefficient between these two members will increase considerably and in an uncontrolled manner so that the movable wedge with its wedge angle of 30.degree. will be in the self-locking region. During rotation of the cutter shaft at its operating speed the movable wedge will press with considerable force with one face thereof against a corresponding face of the cutter shaft and with another face thereof against a corresponding face of the knife holder.
During the machining operation the cutter shaft, together with the elements mounted therein, will heat up considerably, due to the high friction between the wood to be machined and the cutter shaft. Due to the thereby resulting expansion, the movable wedge will, under the action of the centrifugal force, slightly move further in radial outward direction. Due to the self-locking of the wedge the latter will retain this position when the cutter shaft cools down again. During shrinking of the body of the cutter shaft extremely high forces will be imparted by the wedge over the knife holder onto the cutting knife, which will finally lead to the described outward bulging of the material of the cutter shaft body rearwardly of the cutting knife.